Jim:

When a battery is overcharged it "gasses" or some say "boils" the water
away. The later term is very incorrect. What happens if the battery is
overcharged (and all battery chargers overcharge a little bit) is that the
extra voltage above the cell voltage is breaking down the water (H20) into
hydrogen and oxygen. That is the gas bubbles you see. To replace the lost
water, you add water only.

Do not add electrolyte, because as others have already said, you will be
increasing the electrolyte concentration more than is desirable.

So, ONLY ADD DISTILLED WATER.

David